The present invention relates to an electroluminescent thin-film matrix structure in accordance with the preamble of claim 1 that facilitates low power consumption as well as the use of such emission filter materials that in general are incompatible with elevated process temperatures necessary during the production of the light-emitting thin-film structure of a display unit.
Electro-optic structures capable of emitting light are characterized by generation of visible emissions achieved by connecting an electric field over two electrodes, whereby light is produced in a phosphor material placed between said electrodes. If the light emission is viewed through one of the electrodes as is customary with electroluminescent and liquid-crystal displays, at least one of the electrodes must be transparent.
Conventionally, electroluminescent displays are of the matrix type, in which light is generated at the cross-points, or picture elements called pixels, of a transparent column electrode and a metallic row electrode of high conductivity. Emitted light is viewed through the glass substrate, because the transparent electrode pattern layer is deposited prior to the deposition of the light-emitting phosphor layer. A typical electroluminescent thin-film structure is diagrammatically shown in FIG. 1. A transparent conductive layer 2, typically of indium-tin oxide (ITO), is deposited onto a glass substrate 1. The layer is patterned appropriately as, e.g., straight parallel electrodes for a matrix display. Next, a thin-film dielectric layer, thin-film phosphor layer and thin-film dielectric layer are sequentially deposited to form a layered structure 3, 4, 5, which performs as the central component of the electroluminescent display. Finally, a metallic thin-film layer 6 is deposited patterned as the column electrodes in a matrix display. The thickness of the individual thin-film layers is generally of the order of 200 . . . 700 nm. In practice, the thin-film structure must be protected from ambient moisture. This is achieved by laminating a protective glass panel to the structure with epoxy, or alternatively, by using glass encapsulation filled with silicon oil or inert gas.
The thin-film structure shown in FIG. 1 is functional in electroluminescent matrix displays currently in production. The structure has, however, at least two profound problems.
In order to minimize power consumption of the display, the conductivity of the transparent column electrode should be maximally high. Practical constraints pose difficulties when attempts are made to achieve a sheet resistivity lower than 3 ohm/square. Typically, the sheet resistivity can even be in excess of 5 ohm/square. Due to this fact, a major portion of power consumption in an electroluminescent matrix display relates to the power losses in the transparent column electrodes.
In principle, the situation could be improved through augmenting the transparent electrode, which is deposited on the substrate glass, by a narrow metallic stripe of high conductivity. Such a solution is, however, hampered by practical problems, because the metallic stripe must be sufficiently conductive, yet narrow enough not to disturb the readability of the display by its width or to interfere with the processing of the subsequently deposited layers by its thickness.
Another weakness of conventional electroluminescent thin-film structures is associated with the implementation of a multicolor display by means of light filters and an electroluminescent structure emitting white light. Here, in order to avoid the parallax effect, the light filters should be placed at a distance not greater than, e.g., 10 . . . 50 .mu.m from the light-emitting phosphor layer. This would necessitate placing the light filters between the glass substrate and the transparent electrode. Consequently, the high process temperature necessary for the production of electroluminescent thin-film structures excludes the use of light filters based on organic materials.